Could you possibly mark space-age as optional?
I have a mod that adds the agricultural tower without needed the space-age mod enabled but still have space age executable [https://mods.factorio.com/mod/sei-agricultural-tower] and another mod that adds the fruit trees, and it would be nice to use this mod. I did test and you do not need to add my mod as a optional dependency, but it did work with the standalone agricultural tower

Okay! Forgot one could have agricultural towers without space-age mod, I'll added that to the todo list.

Thanks